Okay lets do this again. There are a thousand posts about xerath and his need for help from riot. Be it his passive or his mechanics themselves, xerath is not in a good place. I have posted many times on many xerath threads because like a lot of us here, I freaken love xerath. I am also im the process of making a rather extensive analysis of what I believe to be wrong with him, though I haven't posted it yet.
People saying Xerath's passive is ****: You are wrong. Its a phenomenal passive. Free armor while you build damage is pretty awesome.
People saying that the passive is good with the rise of ad casters, you are also wrong. The highest early game AP i have seen is 35 which would give you a whole 5 armor. Now as a caster, you are generally not running any other armor anywhere (ie not in masteries nor runes) so the ad caster that is building for armor pen, going to build a last whisper and perhaps a black cleaver on top of that, isn't going to care about your passive in the slightest. They will always be able to build up more damage and armor pen that you will armor based off this passive.
The bigger problem is that as a xerath, if you are actually utilizing this passive, you have already failed in your role. Xerath's game is his long range, if someone has closed the gap and you are forced to say good thing i have all that armor, you should have died. This passive would be excellent on a champion like diana or rumble, who needs/wants to be in the center of team fights. It should not be on a champion that is suppose to be fighting out of range of every other champion in the game.
That being said, there are a lot of other problems with xerath, enough that when compared to a champion like, i dont know, lux, makes you question why you are playing xerath. Lux is everything xerath wishes he could be, without nearly as many drawbacks.